The lack of known copies of Sir Orfeo may be due in part to the fact that Tolkien's name does not appear in it, so very few people would know what they were looking at.

I think that the other reason the Sir Orfeo tends to get over looked is because it is an edited text rather than an original composition.

I only know of one copy offered for sale and that was quite some time ago.

BTWMy article at TolkienBooks.net on Songs for the Philologists was updated recently - it now has a complete listing of the contents, a couple of new images and few extra snippets of information supplied by the lucky owner of one copy.